Co-deposition Of Deuterium Research Articles

Elemental Characterization of Neutron-Irradiated Tungsten Using the GD-OES Technique

In reactor-relevant fusion divertor conditions, tungsten (W) will be used as an armor material due to its excellent thermal properties. It will be exposed to impurities from numerous sources, including ion implantation and mixing, neutron transmutation, low-Z plasma-facing-component (PFC) redeposition and codeposition of deuterium and tritium fuel, and trapped helium bubbles. The impurity plasma material–interaction effects are a concern because they can cause gradual degradation of the material and of plasma performance due to dust formation, fuel retention, and even changes to the thermal and mechanical properties of the W armor. It is crucial to measure the amount of impurities in W, and the glow discharge–optical emission spectroscopy (GD-OES) technique is exceptionally well suited for analysis of irradiated samples. GD-OES can measure a sample’s elemental composition by sputtering the surface of the sample, ionizing the eroded material, and measuring the optical emission of the excited atoms. In order for the GD-OES technique to be applied to neutron-irradiated tungsten samples, a mounting system for miniature samples was designed. The sample mounting and centering procedure was successful in measuring the depth distribution of control W and W alloy sample elemental concentrations. These control depth spectra will be used as elemental references for postirradiated samples. The residence time of surface layers was measured, a comparison of signals from different anodes was completed, and the influence of initial surface roughness or nonuniformity was understood. The depth distribution of an arc-welded W-0.4% rhenium (Re) alloy was measured to have a stable Re signal that was distributed evenly in the W matrix. The methods developed here will allow for quantification of impurities and transmutation amounts in neutron-irradiated W. GD-OES is a powerful tool but requires calibration and careful optimization of the parameters to obtain meaningful results.

Material deposition on inner divertor quartz-micro balances during ITER-like wall operation in JET

The migration of beryllium, tungsten and carbon to remote areas of the inner JET-ILW divertor and the accompanying co-deposition of deuterium has been investigated using post-mortem analysis of the housings of quartz-micro balances (QMBs) and their quartz crystals. The analysis of the deposition provides that the rate of beryllium atoms is significantly reduced compared to the analogue deposition rate of carbon during the carbon wall conditions (JET-C) at the same locations of the QMBs. A reduction factor of 50 was found at the entrance gap to the cryo-pumps while it was 14 under tile 5, the semi-horizontal target plate. The deposits consist of C/Be atomic ratios of typically 0.1–0.5 showing an enrichment of carbon in remote areas compared to directly exposed areas with less carbon. The deuterium retention fraction D/Be is between 0.3 and 1 at these unheated locations in the divertor.

Characterization of deuterium retention and co-deposition of fuel with lithium on the divertor tile of EAST using laser induced breakdown spectroscopy

A laser induced breakdown spectroscopy (LIBS) system has been developed to measure and monitor the composition evolution on plasma facing materials (PFMs) of Experimental Advanced Superconducting Tokamak (EAST). As a necessity and important proof of principle experiment, LIBS analysis has been performed for lithium–deuterium co-deposition layer diagnosis of EAST divertor tiles in lab experiments. The distribution of deuterium retention has been obtained from the depth of 0.5–4μm in the divertor tiles. The deuterium/hydrogen concentration ratio was estimated as 0.17±0.02 in lithium–deuterium co-deposition layer. Moreover, the depth profile behaviors of lithium and deuterium indicate that the deuterium retention in divertor tile came from lithium–deuterium co-deposition processes during deuterium discharge in EAST. This work would improve the understanding of deuterium retention and lithium–deuterium co-deposition mechanism and give a guidance to optimize the LIBS system which will be a unique and useful diagnostic approach in EAST 2014-campaign.

Deuterium Retention and Desorption Behavior of Co-Deposited Carbon Film Produced in Gap

Co-deposition of deuterium with carbon in an opening on a plasma-facing surface, a so-called ‘gap’, was simulated by using a deuterium arc discharge with carbon electrodes. The carbon deposition distribution and deuterium retention/desorption behavior of the carbon film were investigated. The amount of deposited carbon decreased exponentially with an increase of the distance from the gap entrance and more rapidly decreased with an increase in discharge gas pressure. The deuterium concentration in the carbon film increased with discharge gas pressure. At a high discharge gas pressure of 36 Pa, the atomic ratio of D/C in the carbon film reached as high as 0.9. Deuterium retained in the film desorbed mainly in the forms of D2, HD, CD4 and C2D4. The desorption behavior of retained deuterium depended on D/C. In a film with a high D/C ratio, desorption of D2 started at lower temperatures. The amount of desorbed hydrocarbons (CD4 and C2D4) increased with D/C. Carbon film with high D/C tended to contain a polymer-like structure, which could be related to the desorption behavior of the retained deuterium. c

Insight into the co-deposition of deuterium with beryllium: Influence of the deposition conditions on the deuterium retention and release

A systematic study of the influence of the deposition conditions on the deuterium retention by and release from co-deposited beryllium layers has been carried out in PISCES-B. Experimental parameters such as the beryllium deposition rate, the incident particle energy and the substrate temperature are shown to affect the level of hydrogen isotope retention in the layers. In addition, the pressure during deposition and the presence of Ar in the plasma are monitored since they influence the layer morphology and the ease of fuel removal by thermal desorption. Consequences for both the tritium inventory and effectiveness of tritium removal during bake-out procedures in ITER will be discussed.

Spatially resolved charge exchange flux calculations on the Toroidal Pumped Limiter of Tore Supra

A spatially resolved calculation of the charge exchange particle and energy fluxes on the Toroidal Pumped Limiter (TPL) of Tore Supra is presented, as a first step towards a better understanding and modelling of carbon erosion, migration, as well as deuterium codeposition and bulk diffusion of deuterium in Tore Supra. The results are obtained with the EIRENE code run in a 3D geometry. Physical and chemical erosion maps on the TPL are calculated, and the contribution of neutrals to erosion, especially in the self-shadowed area, is calculated.

Particle control in long-pulse H-mode discharges of JT-60U

An H-mode plasma was successfully maintained for∼30 s at a constant density of by using divertor pumping even under the outgassing condition. At a high density of , an H-mode plasma was maintained for ∼25 s with a continuous fuel retention, which was ascribed to low outgassing, high co-deposition of deuterium with carbon and the dynamically changed fuel retention process. It was found that the third process worked with the change in edge plasma parameters, in particular particle flux, so as to maintain the electron density against the density control: the outgassing dominated when the divertor pumping was on and the fuel retention dominated when the divertor pumping was off.

DiMES studies of temperature dependence of carbon erosion and re-deposition in the lower divertor of DIII-D under detachment

A strong effect of a moderately elevated surface temperature on net carbon deposition and deuterium co-deposition in the DIII-D divertor was observed under detached conditions. A graphite DiMES sample with a 2 mm wide, 18 mm deep gap lined with silicon catcher plates was exposed to lower-single-null (LSN) L-mode plasmas first at room temperature, and then pre-heated to 200 °C by a built-in electrical heater. At the elevated temperature, deuterium co-deposition in the gap was reduced by an order of magnitude. At the plasma-facing surface of the pre-heated sample net carbon erosion was measured at a rate of 3 nm s−1, whereas without pre-heating net deposition is normally observed under detachment. In a related experiment three sets of molybdenum mirrors recessed 2 cm below the divertor floor were exposed to identical LSN ELMy H-mode discharges. The first set of mirrors exposed at ambient temperature exhibited net carbon deposition at a rate of up to 3.7 nm s−1 and suffered a significant drop in reflectivity. In contrast, two other mirror sets exposed at elevated temperatures between 90 and 175 °C exhibited practically no carbon deposition and their optical reflectivity in the wavelength range above 500 nm was essentially preserved.

Particle balance under global wall saturation in long-pulse discharges of JT-60U

During 30s-ELMy H-mode discharges, the wall-pumping rate decreases with a decay constant of several seconds and then becomes constant. During the constant wall-pumping phase, in discharges with a density of 65% of the Greenwald density, the wall-pumping rate is negative, in other words, outgassing. It has been found that this outgassing rate correlates with an increase in the tile temperature around the outer strike point. In discharges with a density of 80% of the Greenwald density, the wall-pumping rate is positive. Unless a high net deposition rate (>60%) of hydrocarbon is assumed, the positive wall-pumping rate cannot be explained only by the co-deposition of deuterium with carbon even if the outgassing rate is assumed to be zero. The vessel deuterium inventory decreases by 1.1×1024 on one experimental day with 17 long discharges. The main chamber wall is suggested as the deuterium source for the decrease of the inventory.

Co-deposition of deuterium with silicon doped carbon

The co-deposition of deuterium with silicon doped carbon for silicon concentrations between 0–100 at.% in the temperature range from room temperature to 1000 K has been investigated. The eroded material from various different targets was caught on collectors together with the reflected D to build up the co-deposited layers, which were analysed with MeV ion beam techniques. The amount of trapped D per re-deposited target atom depends weakly on the Si concentration. The maximum of about 0.7 D/(Si + C) was found at Si/C ≈ 1. For pure C and pure Si the D concentration is about 0.45 and 0.5 D atoms per re-deposited target atom at room temperature, respectively. For increasing deposition temperature the D concentration does not decrease significantly until about 600 K. At about 1000 K the D concentration for pure carbon layers is still about 30% of the concentration at room temperature. Also, co-deposited layers of stainless steel and of titanium–carbon mixtures were investigated.

Hydrogen isotope inventories in the ASDEX Upgrade tungsten coated divertor tiles

The total hydrogen isotope inventories in the tungsten coated divertor tiles of ASDEX-Upgrade were measured by thermal desorption spectroscopy (TDS) and – in the near surface region – by nuclear reaction analysis (NRA). The highest amounts of deuterium (⩽5 × 10 22 D/m 2) are measured in the inner divertor region outside the separatrix due to the formation of a C:H layer on the tungsten by codeposition of deuterium with carbon. The inventories in the outer divertor – where erosion dominates – are generally smaller by an order of magnitude.

Codeposition of hydrogen with beryllium, carbon and tungsten

The trapping of energetic deuterium codeposited with beryllium, carbon and tungsten has been measured on a silicon collector probe at room temperature. The subsequent release of the trapped deuterium at elevated temperatures was determined by thermal desorption spectroscopy. At room temperature, deuterium codeposits both with carbon and BeO with a ration of 0.41 D-atorns/C-atom and 0.38 D-atoms/BeO, respectively. No codeposition of deuterium with tungsten is observed. The thermal release of codeposited hydrogen from BeO begins at 400 K. All hydrogen is released at temperatures above 800 K.